- Beginners - Haskell.org

System.IO.withFile
by Rafael Gustavo da Cunha Pereira Pinto 20 Jan '09

20 Jan '09

Hi folks, I am trying to use the withFile function of System.IO, but it always return an empty string when testing. Could someone explain why: main= do h<-openFile "test.cir" ReadMode c<-hGetContents h print c > runhaskell test1.hs > "* Teste\n\nR1 1 0 10\nC1 1 0 10uF\nI1 1 0 1mA\n\n.DC \n.PRINT\n" works and main= (withFile "test.cir" ReadMode hGetContents) >>= print > runhaskell test1.hs > "" don't? Thanks! -- Rafael Gustavo da Cunha Pereira Pinto Electronic Engineer, MSc.

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Printf and bool
by Aditya Mahajan 19 Jan '09

19 Jan '09

Hi, I like the printf function from Text.Printf to display results on the terminal. I noticed that there is no format specifier for Bool. Can someone suggest how to create an instance of PrintfArg for Bool? Aditya

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MULTICONF-09 final call for papers
by John Edward 16 Jan '09

16 Jan '09

MULTICONF-09 final call for papers The 2009 Multi Conference in Computer Science, Information Technology and Control systems and Computational Science and Computer Engineering (MULTICONF-09) (website: http://www.PromoteResearch.org) will be held during July 13-16 2009 in Orlando, FL, USA. We invite draft paper submissions. The event consists of the following conferences: · International Conference on Artificial Intelligence and Pattern Recognition (AIPR-09) · International Conference on Automation, Robotics and Control Systems (ARCS-09) · International Conference on Bioinformatics, Computational Biology, Genomics and Chemoinformatics (BCBGC-09) · International Conference on Enterprise Information Systems and Web Technologies (EISWT-09) · International Conference on High Performance Computing, Networking and Communication Systems (HPCNCS-09) · International Conference on Information Security and Privacy (ISP-09) · International Conference on Recent Advances in Information Technology and Applications (RAITA-09) · International Conference on Software Engineering Theory and Practice (SETP-09) · International Conference on Theory and Applications of Computational Science (TACS-09) · International Conference on Theoretical and Mathematical Foundations of Computer Science (TMFCS-09) The website http://www.PromoteResearch.org contains more details. Sincerely John Edward Publicity committee

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Is complete decoupling and abstraction possible in Haskell?
by Tomer Libal 16 Jan '09

16 Jan '09

Hi, I must add that although in general Theorem provers are very performance oriented, we are not interested in performance in our prover as it is used mainly for testing different refinements and extensions. Therefore, we consider haskell as well (as c++). Also, is there a big reference project, implemented in Haskell, that exhibits some of the OO concepts like decoupling, modulation and abstraction? I understand functional programming has other core values like ease of programming and easy validation of correctness but is it also suitable for large structured projects worked on by many programmers (using decoupling for example)? I have read the previous thread about Monads with great interest. The abstration ideas contained in Wadler's papers gave me hope that there is a way to do a full decoupling and abstraction in Haskell, but I have no idea how. I am interested in building a completely generic resolution theorem prover that all of its parts can be implemented separately. In fact, we already have this prover programmed in c++ but I would be glad if I can reprogram it in Haskell for further extensions of the different parts. The idea of the resolution prover is to get a set of clauses in clasual logic and to try and refute them by using resolution. Some of the parts that need to be decoupled are: - Refinements which are datastructures that return the pair (or more) of clauses each time that need to be processed. They are chosen according to some heuristic, i.e. smallest clauses, etc. Each Refinement instance should be able to return the next pair to be processed according to its inner state and heuristics. - Resolvers that process two clauses and tries to unify a specific literal in each. The abstraction here deals with each resolver processing in a different way. Some do factorization and some modulation and each one of this processes have several kinds that should be decoupled as well (i.e. modulator that works by a brute force BFS or

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gtk2hs, inputAdd and forked process
by Johann Giwer 15 Jan '09

15 Jan '09

Some times ago I wrote a frontend for mpg123 using python and gtk2. Now I'm trying to do the same in haskell, but my attempt failed in a very early stage. I like to start a subprocess and read its output via 'inputAdd'. The (simplified) code looks like this: module Main (Main.main) where import System.IO.UTF8 hiding (putStr, putStrLn, print) import qualified Control.Exception as E import System.Glib.MainLoop import System.Environment import System.Posix.Process import System.Posix.IO import System.Posix.Types import Graphics.UI.Gtk main :: IO () main = do name <- getProgName argv <- getArgs case argv of [file] -> play file _ -> putStrLn $ "Usage: " ++ name ++ " MP3 FILE" play :: FilePath -> IO () play file = do initGUI (r0,w0) <- createPipe (r1,w1) <- createPipe pid <- forkProcess $ do closeFd w0 closeFd r1 dupTo r0 stdInput dupTo w1 stdOutput executeFile "mpg123" True ["--output", "alsa", "--remote"] Nothing closeFd w1 closeFd r0 window <- windowNew onDestroy window mainQuit button <- buttonNew set button [ buttonLabel := "Play" ] onClicked button $ do w <- fdToHandle w0 E.handle ( \e -> print e ) $ do hPutStrLn w ( "L " ++ file ) return () set window [ containerChild := button ] widgetShowAll window inputAdd (fromIntegral r1) [ IOIn,IOPri ] priorityHigh ( readData r1 ) mainGUI readData :: Fd -> IO Bool readData h = do E.handle ( \e -> print e >> return True ) $ do (s,_) <- fdRead h 1 putStr s return False When loading the file in ghci, I get half a second of sound and a couple of status message lines. Compiled with ghc, this program gives absolute no output (So threading seams to be the problem?). In a next step, I wrapped every IO action in the main process in a call of 'forkIO' and used MVars for the file name and descriptors, but that doesn't help. Does anybody have experience with 'inputAdd' and forked processes? Thanks in advance -Johann

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Odd behavior from WinHugs
by David Schonberger 14 Jan '09

14 Jan '09

Hi all. I'm attempting exercise 4.6 from Daume's Yet Another Haskell Tutorial: Exercise 4.6 Write a datatype Tuple which can hold one, two, three or four elements, depending on the constructor (that is, there should be four constructors, one for each number of arguments). Also provide functions tuple1 through tuple4 which take a tuple and return Just the value in that position, or Nothing if the number is invalid (i.e., you ask for the tuple4 on a tuple holding only two elements) For starters, I began with the following, stored in a file called MyTuple.hs: data myTuple a b c d = my4Tuple a b c d | my3Tuple a b c | my2Tuple a b | my1Tuple a deriving (Show) When I load this into WinHugs I get the following error: file:.\MyTuple.hs:1 - Syntax error in data declaration (unexpected symbol "a") However I get the same error if I reduce it to a single data constructor, in an effort to create just a quadruple: data myTuple a b c d = my4Tuple a b c d What is even more odd is that I have the following code in a file called Pair.hs: {-data Pair a b = Pair a b deriving (Show)pairFst (Pair x y) = xpairSnd (Pair x y) = y data Quadruple a b c d = Quadruple a a b b deriving (Show)fstPairofQuad (Quadruple a b c d) = [a,b]sndPairofQuad (Quadruple a b c d) = [c,d]-} data Quad a b c d = QuadFour a b c d | QuadThree a b c | QuadTwo a b | QuadOne a deriving (Show) {-fstPairofQuad (QuadFour a b c d) = (a,b)mdlPairofQuad (QuadFour a b c d) = (b,c)sndPairofQuad (QuadFour a b c d) = (c,d)fstLstofQuad (QuadFour a b c d) = (a,d)-} Whether I just keep Quad visible in the file or I take away both pairs of comment braces, when I load that file into WinHugs it works fine, and I have no trouble constructing a Quad or using any of the QuadFour utility fcns (which, admittedly are not what is called for in the exercise... but that's probably besides the point). What in the world might be causing the error in the MyTuple file? Thanks, David

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The problem with Monads...
by Rafael Gustavo da Cunha Pereira Pinto 14 Jan '09

14 Jan '09

Last night I was thinking on what makes monads so hard to take, and came to a conclusion: the lack of a guided tour on the implemented monads. Let's take the Writer monad documentation: all it says is: Inspired by the paper "Functional Programming with Overloading and Higher-Order Polymorphism", Mark P Jones (http://web.cecs.pdx.edu/~mpj/pubs/springschool.html) Advanced School of Functional Programming, 1995. SO WHAT? The best approach is the Part II of the "All About Monads" tutorial. There you have the almost ideal approach, except that the examples are just thrown there, with no step-by-step explanation. Of course one could copy, paste and run it, but this gives pretty much a "is it right?' feeling. Questions like "if a Reader is an application, why don't use a regular function instead?" or "what bind means for a State monad?". I will try to work on a "Part II" extended version on my vacations... maybe a WikiMonad... or MonadPedia... :-) After all, it is my duty as a haskell noob to write another monad tutorial! :D Cheers! -- Rafael Gustavo da Cunha Pereira Pinto Electronic Engineer, MSc.

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First request for references
by Alan Cameron 13 Jan '09

13 Jan '09

Hi, I stumbled across Haskell while researching 'functional programming'. I have done some initial reading and have the following questions: 1. Are there any references to real-world applications using this language? I ask because most of the articles I have found seem to be of an extreme academic nature. I want to be sure that my time will not be wasted delving deeper. 2. Are there any tutorials with full code included which I can use on the HUGS system I have installed? I have found a few "A Gentle Introduction to Haskell 98" which assumes far too much previous knowledge to be immediately useful to me. Thanks for any replies. Alan Cameron

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Referring to several constructors at once
by Tomer Libal 12 Jan '09

12 Jan '09

Hello, I am trying to define a logical calculus using one data type for the rules. There are three types of rules and each has 0,1 or 2 assumptions (rule, rule1 and rule2 below). I have defined all the rules as different constructors so the difference between the types according to 0,1 or 2 assumptions is very weak. I would still like to be able to group the rules in types according to the number of assumptions in order to use pattern matching. Is there a simple way to do that or another way I should implement the data type such that I can refer to the rules both according to their number of assumptions and according to their type? 27 data Rule = Axiom {lowseq :: Sequent} 28 | WeakeningL {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 29 | WeakeningR {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 30 | ContractionL {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 31 | ContractionR {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 32 | PermutationL {rule :: Rule, lowseq :: Sequent} 33 | PermutationR {rule :: Rule, lowseq :: Sequent} 34 | Mix {rule1 :: Rule, rule2 :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 35 | NotL {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 36 | NotR {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 37 | AndL {rule1 :: Rule, rule2 :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 38 | AndR {rule1 :: Rule, rule2 :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 39 deriving (Eq, Show) Thanks, Tomer

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untilM and scanM
by Jan Snajder 11 Jan '09

11 Jan '09

Hi, is there a reason why there is no monadic version of "until" in the Haskell libraries? It would be defined as follows: untilM :: (Monad m) => (a -> Bool) -> (a -> m a) -> a -> m a untilM p f x | p x = return x | otherwise = f x >>= untilM p f The same applies to scanM, also not part of the libraries: scanM :: (Monad m) => (a -> b -> m a) -> a -> [b] -> m [a] scanM f q [] = return [q] scanM f q (x:xs) = do q2 <- f q x qs <- scanM f q2 xs return (q:qs) I often find myself in need for these. To me these seem idiomatic enough to be included in the library. But since they is not, I guess there must be another, more idiomatic way to do this. Thank you, Jan

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